1. Field of the Invention
The present invention relates to an active matrix type display device.
2. Description of the Related Art
As an active matrix type display device, a liquid crystal display device has been known, for example.
As an example of such an active matrix type display device, there has been known a display device in which a plurality of gate lines which extend in the x direction and are arranged in parallel in the y direction and drain lines which extend in the y direction such that the drain lines cross the gate lines and are arranged in parallel in the x direction are formed on a substrate so as to form a matrix. Then, pixels having active elements which are connected to the gate lines and the drain lines corresponding to this matrix are arranged in a matrix array. Scanning signals are supplied to the gate lines from a scanning driving circuit so as to turn on the active elements of the pixels. On the other hand, video signals are applied to the drain lines from a video signal driving circuit and the video signals are written in the pixels through the active elements in the ON state and a display is performed in response to these signals. With respect to a liquid crystal display device, in general, thin film transistors (TFT) are used as the active elements and the video signals are written in pixel electrodes.
Here, when amorphous silicon (a-Si) is used as a material for forming semiconductor layers of the thin film transistors, the scanning driving circuit and the video signal driving circuit are provided as driver ICs which constitute separate parts. On the other hand, when polycrystalline silicon (polysilicon, p-Si) is used as a material for forming the semiconductor layers of the thin film transistors, there has been known a technique in which the whole or portions of the scanning driving circuit and the video signal driving circuit are not provided as separate parts but are integrally formed on a substrate of a display panel on which the pixels are formed.
FIG. 21 is a view showing one example of a conventional scanning driving circuit. The scanning driving circuit 300 includes a shift register 301, level shifters 302 and buffers 303. These elements are integrally formed on a substrate using polysilicon thin film transistors. Outputs at respective stages of the shift register 301 are respectively subjected to the level conversion by the level shifters 302 and are applied to respective gate lines GLn, GLn+1, . . . through the buffers 303 as scanning signals. Each level shifter 302 is constituted of a CMOS (complementary) circuit. The CMOS circuit is a circuit in which both a P channel MOS transistor (hereinafter simply referred to as PMOS) and an N channel MOS transistor (hereinafter simply referred to as NMOS) are present in a mixed state.
As literatures relevant to this prior art, there exists JP-A-2000-305504, for example. This literature describes a driving circuit integrated liquid crystal display device which integrally forms a vertical driver corresponding to a scanning driving circuit and a horizontal driver corresponding to a video signal driving circuit on a substrate using polysilicon thin film transistors. Here, the vertical driver is constituted of a shift register, a level shift circuit and a buffer. The level shift circuit is formed of a CMOS circuit including a CMOS latch cell and a CMOS inverter.
Further, besides the above-mentioned literature, as a literature which is relevant to the scanning driving circuit, there exists JP-A-5-243577. This literature describes a display device in which a driving circuit at a gate line (gate wiring) side and a driving circuit at a data line (drain wiring) side for active matrix are built in (integrally formed in) the inside of a substrate. The scanning driving circuit is not formed of a CMOS circuit and thin film transistors used in the display device are constituted of polysilicon thin film transistors of either N channel or P channel. Further, a level shifter circuit and a buffer are not provided to the display device. A shift register is constituted of a shift register cell, while the shift register cell is constituted of four transistors and one bootstrap capacitor.
Further, in the above-explained JP-A-5-243577, an example which uses a shift register capable of performing a bootstrap operation as a data line side driving circuit is described. A shift register cell is constituted of a bootstrap capacitor and three transistors. Outputs from the shift register cell are inputted to a gate of a sample holding transistor. Since the output from the shift register cell is applied to the input of the gate with an amplitude which is substantially twice as large as a clock signal due to the bootstrap operation, the sample holding transistor is switched at a high speed.
Besides the above-mentioned literatures, in the JP-A-62-66291, one example of a video signal driving circuit is described. That is, in a thin film scanning circuit in which gates of analogue switches which are constituted of thin film transistors are sequentially selected by a gate selection circuit formed of a complementary (CMOS) thin film transistor, a first electrode of a MOS type capacitor having the same structure as the above-mentioned thin film transistor is connected to a gate of the above-mentioned analogue switch, and a second electrode of the MOS type capacitor is connected to one of internal terminals of the thin film scanning circuit. The gate selection circuit is a logic circuit using a complementary TFT and is formed of a shift register or the like, for example. Here, the gate of the gate selection circuit does not imply a gate of the gate line of the active matrix panel but implies a gate which is served for selecting a gate of a analogue switch in the inside of the video signal driving circuit. Accordingly, the gate selection circuit is not used as a scanning signal driving circuit but is used as a video signal driving circuit. Due to such a constitution, the gate of the analogue switch has a voltage thereof elevated to a value which is approximately twice as large as a power supply voltage due to a bootstrap effect of the MOS type capacitor so that a load driving ability of the analogue switch is increased. Accordingly, video signals are applied to video signal lines (corresponding to drain lines) through the analogue switches in the ON state.
An example of another video signal driving circuit is described in JP-A-5-281517. In this literature, the video signal driving circuit is constituted of a switch served for transferring video signals and a circuit which drives the switch. The circuit which drives the switch includes a shift register and booster means which boosts an output of the shift register. Here, the booster means is formed of a transistor, a capacitive element and a diode. Due to such a constitution, while a power supply voltage applied to respective transistors in the inside of the shift register and a booster circuit is being held at a low voltage of 7V, a high voltage of 12.3V can be generated so that signals (video signals) having an amplitude of 11V can be handled. Here, the booster means uses two or more shift register outputs with respect to one switch and, at the same time, the same shift register output is used in common partially by a plurality of switches. Further, as thin film transistors, NMOS transistors or PMOS transistors are used. Still further, in this literature, there is a description that active elements, switches for transfer, the shift register and the booster means are preferably integrally formed on a same base body (on a same substrate) as a semiconductor integrated circuit.
An example of still another video signal driving circuit is described in JP-A-2000-275611. In this literature, a video signal driving circuit is constituted of a driver IC which is arranged on a tape carrier as an external circuit of an LCD panel and a time-division switch which is formed on the LCD panel. Pixel signals (video signals) are outputted as time-sequential signals corresponding to the number of time-division (3 in this embodiment) from the driver IC. Then, these time-sequential pixel signals are sampled by time-division using time-division switches and are supplied to signal lines corresponding to the pixel signals (drain lines corresponding to R, G, B in this literature). Due to such a constitution, it is possible to reduce the number of output pins of the driver ICs compared to the number of drain lines. Analogue switches are used as the time-division switches. A set of time-division switches are formed of transmission switches having the CMOS constitution which are constituted of three PchMOS transistors and three NchMOS transistors. A set of time-division switches are formed on the same substrate as an LCD panel using polysilicon TFTs. These are controlled by six control lines in total consisting of three selecting signals and three inverting signals of selecting signals.
However, the devices described in these literatures have following drawbacks.
First of all, with respect to the scanning driving circuit, in FIG. 21 and JP-A-2000-305504, the CMOS circuit is used in the level shift circuit. Accordingly, it is necessary to form both PMOS transistor and NMOS transistor in the manufacturing steps and hence, the number of processes is increased. With respect to the gate side driving circuit described in JP-A-5-243577, the CMOS circuit, the level shift circuit and the buffer are eliminated by using the bootstrap capacitor in the shift register. However, to the gate lines (gate wiring), as the scanning signals, signals which elevates the voltage once and, thereafter, elevates the voltage again to another stage is applied.
Further, with respect to the video signal driving circuit, JP-A-2000-275611 describes the example which uses the time-division switches. However, since the time-division switches adopt the CMOS constitution and hence, the number of manufacturing processes is increased. This literature describes that the transmission switches having the PMOS or NMOS constitution may be used. However, no specific constitutional examples are described in the publication and hence, it is considered that the transmission switches are controlled by three control lines instead of six control lines in this case. However, in case that the PMOS or NMOS constitution, that is, the single channel constitution is adopted, when the voltage of control lines inputted to the gates of analogue switches used in the time-division switches and the voltage of video signals are close, there arises a problem that the voltage of video signals is changed in front of and behind the switches due to the resistance of the transistors.
Further, with respect to the video signal driving circuit, JP-A-5-243577, 62-66291 and 5-281517 describe examples in which the video signal driving circuit utilizes a bootstrap effect when analogue switches are turned on. However, these literatures fail to describe the time-division switches and assumes the use of the shift register as a presumption so that the combination of the shift register and the time-division switches is also not taken into consideration.
Accordingly, it is the first object of the present invention to provide a display device having a scanning driving circuit which ensures high degree of freedom in designing waveforms of scanning signals while reducing power consumption.
It is the second object of the present invention to provide a display device having a video signal driving circuit with small power consumption which can apply video signals having least degradation to drain lines while reducing the number of output pins of driver ICs.
It is the third object of the present invention to provide a display device requiring the least number of manufacturing processes.